1. Field of the Invention
This invention relates to directional couplers and, more specifically, to a directional coupler which has substantial immunity to misalignments.
2. Brief Description of the Prior Art
A directional coupler is a four port device which receives power through one port and divides the power between two other ports with the fourth port being isolated and receiving very little power due to cancellation of signals within the coupler. The coupling may be weak or strong wherein, in the case of weak coupling, for example, perhaps only ten percent of the power received will be sent to the coupled port whereas in the case of strong coupling, for example perhaps fifty percent (3 dB) of the received power will be sent to the coupled port.
Directional couplers can be fabricated in many types of embodiments, one such type of embodiment being as a part of a multilayer printed wiring board (PWB) or multilayer ceramic thin film network (TFN). In the fabrication of directional couplers as part of a multilayer PWB, the fabrication tolerances for a state of the art process are typically about .+-.0.002 inch (3 sigma) for layer to layer registration of metal patterns. For high frequency applications, for example 1 to 100 GHz, and for thermal considerations, the dielectric layers for a multi-layer PWB or TFN are maintained thin and in the range of from about 0.001 inch to about 0.005 inch. For this range of thicknesses, interlayer registration is critical to yielding highly repeatable directional coupler designs.
For a stripline structure with two centered metal layers between two ground planes, the directional coupler of choice for high values (3 dB) is a broad-side coupled directional coupler. The sensitivity of the particular design to dimensions and layer to layer registration increases with higher coupling values. Couplers of this type generally comprise a pair of closely spaced apart electrically conductive elements which may have a width of about 4 to 6 mils, which are spaced apart by about 1 mil and which have a length of less than .gamma./4.epsilon..sub.r.sup.1/2 where .gamma. is the free space wavelength at the stripline design frequency for the coupler and .epsilon..sub.r is the effective dielectric constant of the coupler dielectric. It can be seen that such couplers which are misaligned by .+-.0.002 inch and which are designed to provide 3 dB of coupling will provide instead about 4.5 dB of coupling. Such errors often cannot be tolerated. Accordingly, an improved directional coupler structure in conjunction with PWBs which is less sensitive to misalignment is highly desirable.